This invention relates generally to automotive vehicle instrumentation systems and, more specifically, to improvements in meters that display information, such as speedometers and tachometers for example. Although the presently preferred embodiment of the invention will be hereinafter disclosed in the context of a combination speedometer/tachometer, some principles are more universally applicable. In a more generic sense, one aspect of the invention involves the use of mechanically operated electrical switches to concurrently program a meter to a particular one of plural signal sources and to an appropriate calibration for that particular signal source. A more specific aspect of the invention as applied to an automotive vehicle speedometer or tachometer involves the ability to reliably assemble the device with an initial predetermined programming of speed signal source selection and calibration that is suitable for certain vehicle models, while enabling the device to be conveniently re-programmed for selection of an alternative speed signal source and different calibration appropriate for such alternative speed signal source. Such re-programming can be performed before the vehicle leaves the factory and thereafter, as may be required.
In today's automotive vehicles, instrumentation clusters must be more versatile because they must adapt to vehicle configurations where the signal source may come from either conventional pick-ups or serial data links, such as the SAE standard J1708 data link used in heavy trucks. In the case of a speedometer/tachometer, many combinations of variables like tire size, differential gearing, number of transmission wheel teeth, and number of flywheel teeth compound the problem of calibrating the incoming speed signals. Change in any one of these parameters affects signal frequency, and so the cluster must calibrate the road speed or engine speed indications based on the specific values of these variables.
From the standpoint of simplifying parts requirements, it is desirable to have a common meter for all vehicle configurations, but this poses a number of questions. Can a single speedometer/tachometer be manufactured that will operate from conventional signal pick-ups or from an SAE 1708 serial data link with the changing of a switch setting? Can the assembly be electronically programmed with calibration data to prevent human error in switch setting? Can the factory preset be verified? Can the customer in the field modify the calibration setting if, for example, the tire size is changed?
The present invention is directed to a novel and unique improvement that enables these questions to be answered in the affirmative.